Linux: allow compile flags to be passed to AC_CHECK_LINUX_BUILD

Linux: allow compile flags to be passed to AC_CHECK_LINUX_BUILD

Some linux autoconf tests require particular compile flags such as
-Werror.  Add a parameter to the AC_CHECK_LINUX_BUILD macro that
lets the caller pass in any needed special flags.

Adapt a few existing tests that were still using AC_TRY_KBUILD
directly.

Change-Id: I4a5e3db74f6fb9c35bfa7b2964a294f646e78626
Reviewed-on: http://gerrit.openafs.org/3769
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>